For a detergent used to remove stains on ceramic products such as bathroom tiles, washing stands, toilet bowls, tiles for interior decoration and the like, an acidic detergent comprising hydrochloric acid, citric acid, etc. as a cleaning component, a neutral detergent or a chlorine-based detergent has been used. Conventionally, for such detergents, studies are being carried out on a water-soluble thickener which works effectively upon being mixed into liquid detergents to suitably adjust the viscosity, so that when the liquid detergent is applied on the vertical side of a tile or on a toilet bowl, the liquid detergent flows down on the cleaning surface slowly.
For instance, among the liquid acidic detergents for cleaning tiles which use sulfamic acid and glycolic acid in combination as acidic base materials, an acidic detergent comprising polyvinyl alcohol as a thickener and urea as a stabilizer is disclosed (JP-A-53-46302). This detergent comprises sulfamic acid instead of hydrochloric acid as an acidic base material in consideration of safety. However, since sulfamic acid alone is less effective than hydrochloric acid in the cleaning effect, the detergent uses glycolic acid as an acidic adjuvant. Further, polyvinyl alcohol as a thickener is likely to result in reducing viscosity due to the occurrence of gelation when used at low temperatures in winter, etc.
Further, detergents which comprise a mineral acid such as hydrochloric acid, a halogen scavenger, a surfactant and a thickener as essential components, and which comprise xanthan gum, cationized cellulose, polyethylene glycol, polyvinyl pyrrolidone or polyacrylamide methylpropanesulfonic acid as a thickener are disclosed (for example, in JP-A-9-143498 or JP-A4-209700). Among these, a homopolymer of polyacrylamide methylpropanesulfonic acid shows a thickening effect to the extent that may be obtained by 10% hydrochloric acid, but it is not satisfactory because it cannot satisfy both the fluidity and the anti-dripping property at the same time. None of these thickeners have a sufficient thickening effect under acidic conditions and satisfactory stability over time. Thus, reports have been made upon studies on using a copolymer instead of a homopolymer as a water-soluble thickener.
For instance, reports have been made that a water-soluble thickener comprising a copolymer comprising (meth)acrylamide alkylsulfonic acid and an alkyl group-containing unsaturated monomer (JP-A-10-279636) and a water-soluble thickener comprising a crosslinked copolymer obtained by the polymerization of-(meth)acryl amide alkylsulfonic acid and a crosslinking monomer with (meth)acrylic acid, dialkyl acrylamide, acryloylmorpholine, hydroxyethyl methacrylate, vinyl pyrrolidone or the like, cause a less viscosity reduction under weakly acidic conditions or in brine solutions (JP-A-9-157130, JP-A-2001-114641, JP-A-2001-115135, etc.). These water-soluble thickeners exhibit some thickening effect and the stability in the thickened solution over time under mildly acidic conditions which cause less irritation on the skin when used in the pharmaceutical products, the cosmetic products or the like. However, their thickening effect is unsatisfactory in strongly acidic detergents such as those comprising about 10% by weight of hydrochloric acid and having a pH value of 1 or less.
Moreover, in the processes of preparing copolymers comprising crosslinking monomers as described in JP-A-9-157130, JP-A-2001-114642, JP-A-2001-115135, etc., there are problems that high molecular weight water-soluble polymers having a weight-average molecular weight of more than 5,000,000 are used, and that since full control of the reaction is difficult, and thus water-insoluble polymers which form a molecular structure in the form of three-dimensional network are likely to be produced.
As such, conventional thickeners have exhibited insufficient thickening effect or stability over time in strongly acidic aqueous solutions. In an attempt to enhance the thickening effect of a detergent, an improvement such as increasing the amount of a thickener to be added may be considered, but this causes the reduction in the fluidity of liquid acidic detergents and makes it difficult for the detergent to flow out from the nozzle of a detergent container, thus making it unpleasant to use the detergents. Further, detergents are produced to comprise large amounts of organic materials, which is environmentally not preferable. Also, on the other hand, when the improvement in the fluidity of acidic detergents is made by adjusting the amount of the thickener to be added, there is a problem in that a sufficient anti-dripping property of the detergent on the ceramic surfaces and the like cannot be obtained.